IOWA ACADEMY OF SCIENCES. 91 



usually easily discriminated, and in practical field work only 

 occasionally lead to confusion. 



Waterlaid Beds. — In general geologic work the record of the 

 past is read in the deposits of the succeeding periods In a 

 large majority of cases these deposits are waterlaid, and each 

 class of waterlaid beds, river, lake and beach, have distinctive 

 characteristics. If, then, waterlaid deposits be found buried in 

 the drift they may show, either by their physical character and 

 distribution, or by their contained fossils, something of the 

 length and prevailing climate of the period in which they were 

 laid down. Unfortunately, perhaps, there is always a consid- 

 erable amount of water action in connection with an ice sheet 

 and large bodies of waterlaid beds, contemporaneous with one 

 stage of the ice, may be buried beneath the drift after a wholly 

 unimportant interval. 



The gravel beds may, and do, grade laterally into the drift, 

 proving their contemporanity. They may also carry large 

 numbers of flattened and striated stones, obviously not long 

 subjected to the wearing action of running water. On the 

 other hand they may be well rounded and water worn and indi- 

 cate deposition at a considerable distance, at least, from the 

 ice front. The gravels, whatever their form and origin, may 

 be fresh, hard and uncemented, or they may be weathered, soft, 

 ferruginated and cemented into conglomerate. Since gravel 

 beds are readily permeable and afford easy channels for under- 

 ground water it may be granted that all the processes indicated 

 might' leave their marks upon a really young gravel. As a 

 matter of fact, however, the gravels found in connection with 

 the Wisconsin drift are almost uniformly fresh, while the 

 Buchanan gravels, and the few which have been referred to 

 the Aftonian quite as uniformly show signs of age. It would 

 seem that this possible source of error is really after all quite 

 unimportant. Furthermore, it has often been shown that in 

 many cases the weathering of the bowlders, both in the gravel 

 and in the older tills, took place after they were glaciated. 



Topographic Changes. — One of the most easily recognized and 

 significant phenomena indicative of differing ages is topo- 

 graphic change. It is true that the rate of development of 

 topography is dependent on several variable factors, and may 

 differ both in relation to position and time, but the elements 

 due to these factors may often be eliminated, and in such cases 

 the topographic differences become probably the best indices 



